Production of silicon iron sheet stock



United States Patent 3,152,021 PRODUCHGN 0F SELHJON EGN SEEET STQZIK Victor W. (Iarpenter and .l'ohn M. Jackson, Middletown, and Robert W. Squibb, Zmesville, Ohio, assignors to Armeo Steel ilorporation, Middietown, Ghio, a corporation of Ghio Filed Feb. 27, 1963, Ser. No. 262,288 7 Claz'uns. (Cl. 148-111) This is a continuation-in-part of the copending application in the names of the same inventors, Serial No. 773,- 419, filed November 12, 1958, now abandoned, and hearing the same title.

The invention pertains to magnetic materials which can be made by strip annealing methods. Much of this type of product is of a non-oriented nature, but aspects of this invention can also be used in the manufacture of oriented products where the particular qualities imparted by the essential sequence of steps taught herein are of importance. The invention will, however, be described in connection with the production of non-oriented silicon iron, without limitation.

By non-oriented silicon iron is meant a product devoid of such a degree of preferred orientation of the crystals as would produce marked differences in magnetic permeability of the product as measured in the rolling direction and in a direction at right angles thereto. Nonoriented silicon irons are of utility in the manufacture not only of rotating electrical machinery, but also in the manufacture of transformers the cores of which are made up of E-shaped, l-shaped, or L-shaped laminations. In such uses rotat ng elements such as motor armature laminae and transformer laminations are formed from the steel sheet stock by means of dies; and the importance of a good die life is obvious.

A primary object of the invention is the provision of a silicon-iron stock of either type which will be non-aging and at the same time characterized by a high die life.

This primary object of the invention and others, which will be set forth hereinafter or will be apparent to one skilled in the art upon reading these specifications, are accomplished by that procedure of which an exemplary embodiment will now be described. Reference is made to the accompanying drawings wherein:

FIG. 1 is a photomicrograph at l000 magnification of a piece of silicon iron sheet stock which has been processed in a typical fashion as hitherto known in the art.

FIG. 2 is a photomicrograph at 1000 magnification of silicon iron stock which has been processed in accordance with this invention with a dry-furnace anneal at a temperature of about 2050 F., and showing a novel surface condition as compared with the product of FIG. 1.

The figures are micro sections made in the standard way, the lower layer being the silicon iron, and the upper layer being a layer of copper or the like. Between the two layers is an indication of the surface condition on the silicon'iron produced by the process steps hereinafter discussed.

In the manufacture of non-oriented silicon iron sheet stock in the past, it has been a common practice to produce thin bar by hot rolling and then subject the thin bar 'to a decarburizing treatment. Thin bar is usually a material about .1 in. in gauge, although the exact thickness is not critical. The thin bar, after decarburizing, was reduced to final gauge, usually by hot rolling; and if originally produced in the form of sheets, the sheets were welded end to end to form a coil suitable for continuous annealing. Various temper rolling and pickling steps might be employed intermediate the rolling of the material to final gauge and the welding step, all preliminary to ilfizfi l Patented Get. 6, 1 964 a final anneal at a relatively high temperature, namely, a temperature of about 2050 F. It is generally understood that, in order to produce the optimum magnetic characteristics which the product is capable of acquiring, and including the optimum core loss characteristics, a high temperature final anneal is necessary.

Products made as above indicated have many desirable characteristics; but they may have erratic core loss behavior generally known as aging, in which the product in instances suffers a chamge in core loss with the passage of time, a phenomenon believed to be due to an instability which gives rise to a tendency to precipitate carbon compounds.

In the pjrocess of the present invention, the material is reduced to gauge in any suitable way. The reduction may be accomplished either by hot rolling or by cold rolling, or by combinations of the two, which is a distinct advantage in processing. The product may be hot rolled to thin bar, and then further hot rolled to gauge, giving a product in sheet form which will require Welding to form it into a coil of indefinite length. But the principles of this invention are equally applicable to a material which is hot rolled from slabs into long coils, and thereafter pickled and cold rolled to final gauge while still in coil form. The final thickness of the material may be substantially in the range of 24 to 29 gauge.

Silicon iron suitable for the practice of the invention, particularly in the manufacture of non-oriented stock, may be defined as a material containing substantially 0.5% to about 3.8% silicon. Aluminum may be present in quantities up to about 0.5%. The carbon content initially is not a limitation on the invention but usually varies from about 0.02% to about 0.08% in accordance with the manner in which the steel has been made. In addition to silicon, aluminum and carbon, the alloy may contain such amounts of other elements, e.g., manganese, phosphorus, sulfur and the like as are usual in silicon irons produced commercially. The balance of the alloy will be substantially all iron. The analysis given above relates to the material after processing has started, i.e., it is not a ladle analysis; but it may be the analysis of the silicon iron immediately before or immediately after the hot rolling.

An exemplary routing for the material in accordance with the present invention may be given as follows:

The material is hot rolled, or hot rolled and cold rolled to final gauge as stated above.

If the material has been hot rolled to final gauge, it will be usual to temper roll it, pickle it and then again temper roll it prior to welding the individual sheets end to end to form a coil. If the material has been cold rolled to final gauge in sheet form, the temper rolling and pickling steps may be omitted, but the individual sheets will then be Welded end to end to form a coil. If the material has been produced in coil form by cold rolling to gauge, the welding step will, of course, be unnecessary. It will be noted that the material is not subjected to a decarburizing treatment at an intermediate gauge.

The material, produced as indicated, is first subjected to a continuous decarburizing anneal at about 1475 F. in a decarburizing atmosphere. Next, the material is treated to a continuous high temperature anneal, preferably at about 2050 F in a dry non-decarburizing atmosphere. It will be found not only that the material is devoid of aging characteristics, but also that it has an excellent die life, as will be hereinafter more fully set forth.

By a decarburizing atmosphere is meant an atmosphere of hydrogen, hydrogen-nitrogen mixtures containing water vapor as the decarburizing agent, or cracked gases such as DX, which, in addition to nitrogen, hydrogen, and carbon monoxide, contain carbon dioxide and water vapor as decarburizing agents. When hydrogen or hydrogen-nitrogen atmospheres are used, a dew point about 125 F1125 is maintained by adding water. When cracked gases are used, the dew point is somewhat lower, since CO is available as an additional decarburizing agent. The annealing treatment can normally be accomplished in one to three minutes at a temperature substantially within a range of 1350 to 1650 F.

By a dry non-decarburizing atmosphere is meant an atmosphere that is substantially non-oxidizing to carbon but is also not carburizing. Such an atmosphere can be hydrogen, nitrogen or mixtures thereof having a dew point less than +50 F. The temperature for the final continuous anneal in the dry atmosphere should be within the range of substantially 1600 to 2200 F. Higher temperatures may be employed if desired but are generally found uneconomical. Permissible variations in temperature will be discussed more at length hereinafter.

In order to trace the development and advantages of this invention, it may be stated that a typical process of making non-oriented silicon iron involved hot rolling ingots or slabs to bars having a thickness of about .109 in. The unpickled bars still carrying the hot mill scale were subjected to a box anneal in order to decarburize them. The product was then pack rolled into sheets of the desired finished gauge, and pickled and temper rolled still in sheet form. The sheets were welded end to end and the resultant strip was subjected to a continuous anneal at about 2050 F. in dry hydrogen. This material was subject to high core loss aging despite the high temperature of the final anneal.

Thus when a silicon iron sheet stock is formed by the typical prior art process outlined above, i.e., by a proce dure involving decarburizing at a thin bar gauge followed by further reduction and a high temperature anneal, a material is produced which not only is subject to aging, but is characterized by relatively poor die life. The surface condition of such a product is illustrated in FIG. 1, which is a photomicrograph of a material rolled from decarburized bars, pickled, welded, and strip annealed at 2050 F. in dissociated ammonia. The dark line in the photomicrograph shows a layer of silica particles coalesced by the high temperature anneal. These silica particles adversely affect die life; and the material of FIG. 1 gave a die life of only about 20,000 strokes.

It was then found that if the final high temperature anneal were carried on in wet decarburizing atmosphere the core loss aging would be improved; but the die life became very bad. Bumps formed very rapidly on the hearth rolls, and the surface skin of the silicon iron strip could be loosened by bending or punching.

Thus a final heat treatment in a wet decarburizing atmosphere will not give a reasonably satisfactory die life on previously pickled material, and within the ranges set forth it is not high enough to develop the optimum magnetic properties of the material. A decarburizing treatment oxidizes silicon, producing at and near the surfaces of the stock a distinct layer comprising a very substantial quantity of silica. In the manufacture of highly oriented silicon iron stocks, it has sometimes been the practice to decarburize just prior to a final high temperature box anneal. Under these circumstances, an annealing separator must be used, generally MgO; and the result is that the finished product is characterized by a glassy coating which, while it may under some circumstances have utility as an interlamination insulator, would be destructive of die life. Such a coating, moreover, is not generally desired on non-oriented materials, and is not always desirable on oriented stocks.

When, however, the particular series of process steps outlined above as characteristic of this invention is followed, a product is produced which is not only non-aging but has remarkable die life characteristics. The surface condition of this product is illustarted in FIG. 2 which shows, at 1000 magnification, the surface of a silicon iron which has been hot rolled to gauge, pickled, welded, treated at 1475 F. in a wet decarburizing atmosphere, and then continuously annealed at 2050 F. in a dry nondecarburizing atmosphere. Surprisingly, the surface of such material exhibits a very thin band of dark oxide at about the mid-thickness of the surface skin. Without prejudice, the theory is that this band or line represents the original location of the interface between the base metal and the skin formed during the decarburizing operation. In the final high temperature anneal in a dry non-decarburizing atmosphere, some slight oxidation of silicon and aluminum took place below this band, raising the band to about the mid-thickness of the final surface skin. Had a continuous thin band of silica been formed at the interface with the base metal during the final high temperature anneal, it is likely that the surface skin would have been subject to peeling. But, as indicated, a slight oxidation has occurred below the original interface layer, the net result being that the original silica skin and additional particles of silica and alumina are embedded in a mastic of grayish material in the photomicrograph, which grayish material is believed in to be iron containing various refractory oxides. Although the final skin has an appearance indicative of the presence of particles of silica, these are in some way cushioned to the extent that the product has a remarkable die life. Moreover, the final skin is non-peeling.

The appearance of the sheet surface of silicon iron stock, made by the prior art procedure and having the characteristics illustrated in FIG. 1, is dark, while the stock of this invention has a lighter silvery visual appearance. The product of this invention will blue very readily when heated in an open flame, indicating an iron surface, while the prior art material blues much less readily. The surface skin on the material of this invention is not loosened by repeated bending of the stock.

The practice of the present invention requires that the decarburizing anneal be kept separate from the final high temperature anneal in the matter of atmosphere. These operations can best be carried on in separate furnaces where the atmospheres can be carefully and independently controlled. It is possible for economic reasons to carry on both operations in a composite furnace structure; but where this is done, the furnace should consist of a first section for decarburizing with a countercurrent flow of the decarburizing gas, a middle transition section wherein the stock can be maintained under a neutral atmosphere and which serves to isolate the decarburizing section from the high temperature section of the furnace, and a final high temperature section characterized by concurrent flow of a dry non-decarburizing atmosphere, as herein defined. Precautions should be taken against mixing of the atmospheres in the composite furnace except in the transition section, from which the mixed gases can be exhausted.

Example A silicon iron stock containing 3.0% silicon was hot rolled to final gauge in sheet form. It was then temper rolled, pickled, and again temper rolled, after which the sheets were welded together so as to form a coil for continuous annealing. The coil was treated at 1475" F. in a decarburizing atmosphere, as herein defined, for about three minutes at temperature. It was then subjected to a strip anneal at 2050 F. in a dry non-decarburizing atmosphere as above defined, the atmosphere also consisting of dissociated ammonia. The duration of the high temperature heat treatment was about two minutes.

The material so treated was non-aging and had a die life of 135,000 strokes as compared with a die life of 20,000 strokes for a material of the same composition processed in accordance with the prior practice set forth herein.

The decarburizing anneal to which the material is subiected has been set forth as an anneal in a wet gas decarburizing atmosphere within a temperature range of substantially 1350 F. to 1650 F. The anneal in the dry non-decarburizing atmosphere has been set forth as a heat treatment within the range of substantially 1600 F. to 2200" F. and has also been referred to as a high temperature anneal. A temperature for the second anneal which is definitely higher than the temperature of the decarburizing anneal is preferred; and best results are secured in a temperature range of the anneal in the dry non-decarburizing gas within a temperature range of 1800 F. to 2200 F.

H wever, technological advances in the chemistry of silicon iron have made possible the reductions in the final temperature. Also higher core loss limits have een established for certain grades of non-oriented silicon iron. While it is true that for best magnetic characteristics a temperature within the range of 1800 F. to 2200 F. should be used, the higher temperatures within the range being preferred, remarkable and unexpected improvements in die life can be attained at lower temperatures for the non-decarburizing anneal so long as core loss specifications for the particular material can be met. This is the reason for the over-all temperature range for the second or final anneal being set at substantially 1600" F. to 2200 F. It may be noted that where the emphasis is primarily on die life, subjec 'ng the material to a first wet gas decarburizing treatment at substantially 1350 F. to 1650 F. and then to a heat treatment in a dry non decarburizing atmosphere is productive of highly useful and unexpected increases in die life despite the fact that the temperature of the second mentioned anneal may not necessarily be higher than the temperature chosen for the irst mentioned anneal.

While the process has been described in connection with routings particularly appropriate to the manufacture of non-oriented material, it will be understood that the routing steps prior to the final strip decarburizing and strip annealing procedures may be varied. The sk lled Worker in the art will understand that the production of oriented grades of silicon iron generally involve a plurality of carefully controlled cold rolling reductions with intermediate anneals also of carefully controlled nature, and a final high temperature anneal which may be the strip anneal of this process. Still other procedures involve intermediate high temperature treatments.

Modifications may be made in the invention without departing from the spirit of it. The invention having been described in an exemplary embodiment, what is 6 claimed as new and desired to be secured by Letters Patent is:

l. A process of producing non-aging silicon iron sheet stock having good die life which comprises reducing a silicon iron sheet stock to substantially 24 to 29 gauge by a procedure involving pickling, and subjecting the stock first to a continuous anneal at substantially 1350 F. to 1650 F. in a wet gas decarburizing atmosphere, and then without intervening surface treatment to a continuous anneal at substantially 1600 F. to 2200 F. in a dry non-decarburizing atmosphere.

2. A process of producing non-aging silicon iron sheet stock having good die life which comprises reducing a silicon iron sheet stock to substantially 24 to 29 gauge by a procedure involving pickling, and subjecting the stock first to a continuous anneal at substantially 1350 F. to 1650 F. in a wet gas decarburizing atmosphere, and then without intervening surface treatment to a continuous anneal at substantially 1800 F. to 2200 F. in a dry non-decarburizing atmosphere.

3. The process claimed in claim 2 in which the stock is formed by hot rolling into sheets, the sheets being temper rolled, pickled, and again temper rolled, and Welded together so as to form a coil, all prior to the said continuous annealing treatments.

4. The process claimed in claim 2 wherein the silicon iron stock is reduced to gauge by a procedure involving cold rolling into sheets, the sheets being Welded together into coils prior to the said continuous annealing treatments.

5. The process claimed in claim 2 wherein the silicon iron stock is reduced to final gauge by a procedure involving at least one cold rolling in strip form.

6. The process claimed in claim 2 wherein the decalburizing atmosphere is an atmosphere of a material chosen from a class consisting of hydrogen, hydrogennitrogen mixtures, and cracked hydrocarbon gases, the said atmosphere having a dew point of about F. 1-25" F.

7. The process claimed in claim 2 wherein the decarburizing atmosphere is an atmosphere of a material chosen from a class consisting of hydrogen, hydrogennitrogen mixtures, and cracked hydrocarbon gases, the said atmosphere having a dew point of about '+125 F. i25 F., the dry non-decarburizing atmosphere being of a material chosen from a class consisting of hydrogen, nitrogen, and mixtures thereof, said atmosphere having a maximum dew point of about +50 F.

No references cited. 

1. A PROCESS OF PRODUCING NON-AGING SILICON IRON SHEET STOCK HAVING GOOD DIE LIFE WHICH COPRISES REDUCING A SILICON IRON SHEET STOCK TO SUBSTANTIALLY 24 TO 29 GAUGE BY A PROCEDURE INVOLVING PICKLING, AND SUBJECTING THE STOCK FIRST TO A CONTINUOUS ANNEAL AT SUBSTANITALLY 1350* F. TO 1650*F. IN A WET GAS DECARBURIZING ATMOSPHERE, AND THEN WITHOUT INTERVENING SURFACE TREATMENT TO A CONTINUOUS ANNEAL AT SUBSTANTIALLY 1600*F. TO 2200*F. IN A DRY NON-DECARBURIZING ATMOSPHERE. 